1. Field of the Invention
The present disclosure relates to a circuit breaker, and more particularly, to a neutral pole current transformer module for detecting a neutral pole (also called a neutral phase, N phase, or N pole) current in a three-phase four-wire type or a so-called 4-pole circuit breaker having an overcurrent relay (can be abbreviated as OCR) as a control unit, and a neutral pole current detecting apparatus having such a neutral pole transformer module.
2. Background of the Invention
A neutral pole (in other words “N phase”) refers to a pole (that is “phase”) of an electric line connected to a neutral point when a transformer is Y-connected in a single phase three-wire alternating current or three-phase alternating current grid. Distinguished from a ground line, a neutral pole line (hereinafter, referred to as a “neutral line”) is classified as a voltage line.
In an electric power distribution grid, in addition to line voltages of 3 phases such as R phase, S phase, and T phase, a voltage between the three-phase lines and a neutral line, namely, a phase voltage may be used, and here, the line voltages are mainly used as an electric power to drive an electric motor, while the phase voltage is commonly used to turn on an electric light.
A current flowing in a neutral line is not greater than 20% of a phase current such as an R-phase current, an S-phase current, and a T-phase current, but this considers only an unbalance current. When a nonlinear load (e.g., a rectifier, an inverter, an uninterruptible power supply, a computer, a monitor, a copy machine, etc.) or a load generating harmonics, such as an electric furnace or an electric welder exists, a current flowing in the neutral line is greater than a phase current in many cases. Thus, protection for a neutral pole circuit, namely, a neutral line, is required.
FIG. 1 is a view illustrating a configuration of a protecting device of a neutral pole circuit according to an example of a related art.
In FIG. 1, the protecting device of a neutral pole circuit according to an example of the related art may comprise a Rogowski coil 1a for detecting a neutral pole current and providing the same and a circuit breaker 100.
The circuit breaker 100 comprises three switching contacts 50 for three phases, an overcurrent relay 40, and three phase current detectors 60.
The three switching contacts 50 each comprise a fixed contact and a movable contact having a circuit closing position in which each switching contact is brought into contact with a corresponding fixed contact to close an electric power circuit 2 of each phase (here, an R-phase electric power circuit is designated by reference numeral 2a, an S-phase electric power circuit is designated by reference numeral 2b, and an T-phase electric power circuit is designated by reference numeral 2c) and a circuit opening position in which each switching contact is separated from a corresponding fixed contact to open the electric power circuit 2 of each phase.
The circuit breaker 100 may comprise a switching mechanism (please refer to 50a of FIG. 4) providing mechanical driving power for switching the movable contact to the circuit closing position or the circuit opening position, and a configuration of the switching mechanism may be referred to Korean Patent document KR10-0771918 B1 registered by an applicant of the present invention.
The overcurrent relay 40 is a control unit for controlling a switching operation of the circuit breaker 100. The overcurrent relay 40 is connected to the Rogowski coil 1a and the current detector 60 and receives a neutral pole current and detection signals of the three-phase currents respectively from the Rogowski coil 1a and the phase current detector 60. Also, the overcurrent relay 40 compares values according to the received current detection signals with preset reference values, and when any one of the values according to the received current detection signals is equal to or greater than a corresponding reference value, the overcurrent relay 40 outputs a trip control signal. The trip control signal serves to trigger the switching mechanism to operate to the opening position through a trip actuator (not shown). Then, the movable contact is separated from a corresponding fixed contact by driving force provided from the switching mechanism, thus protecting the neutral pole circuit or the general three-phase circuits and electric load devices connected thereto from a fault current.
The three phase current detectors 60 are configured by Rogowski coils in a low-voltage circuit breaker such as an air circuit breaker.
The Rogowski coil, a device for basically measuring a current by using a change in magnetic flux caused as a current is changed, comprises a core having an annular shape allowing an electric line as a current amount measurement target to pass therethrough and a coil wound around the corresponding core. The Rogowski coil having differential characteristics, provides a 90-degree phase-shifted detection signal, compared with an actual current, as a voltage signal generally in units of millivolt (mV).
Since the Rogowski coil 1a for detecting a neutral pole current is a Rogowski coil, the Rogowski coil 1a has differential characteristics like the phase current detector 60, so it may provide a neutral pole current detection signal 90-degree phase-shifted than an actual current as a voltage signal.
However, a low voltage circuit breaker such as an air circuit breaker has a frame (i.e., a rear enclosure unit formed of an insulating partition of each phase) whose length is different according to magnitudes of a rated voltage (rated current). Thus, a size (width or diameter) of a bus bar corresponding to an electrical conductive terminal unit installed on a rear surface of the frame connected to an electric power source side electric line and a load side electric line is varied. Since the Rogowski coil 1a for detecting a neutral pole current or the phase current detector 60 are installed in the bus bar, the Rogowski coils 1a and the phase current detectors 60 in various sizes should be prepared according to sizes (widths or diameters) of the bus bar. Also, in order to manufacture the Rogowski coils 1a for detecting a neutral pole current, a plurality of metal molds for manufacturing a core and a plurality of insulator molds are required. Thus, the preparation of all the Rogowski coils 1a for detecting a neutral pole current according to the ratings incurs high manufacturing cost, and causes a manufacturing cost of the overall circuit breaker to increase for the manufacturer of the circuit breaker.
Also, while a voltage signal in units of millivolt is being transmitted from the Rogowski coil 1a for detecting a neutral pole current installed to be adjacent to the bus bar, an outer side of the circuit breaker, to the overcurrent relay 40 installed within the circuit breaker, a neutral pole current detect signal may be distorted or mixed due to an influence of external noise, causing the overcurrent relay 40 to malfunction to erroneously break the circuit.
Also, in order to prevent this problem, shielding noise from the Rogowski coil 1a for detecting a neutral pole current to the overcurrent relay 40 installed within the circuit breaker requires a shielding structure employing a high level of shielding technique, increasing cost of the circuit breaker.
Hereinafter, a configuration of a protecting device of a neutral pole circuit according to another example of the related art will be described.
Another example of the related art illustrated in FIG. 2 is different from one example of the related art illustrated in FIG. 1, in that a manufacturer of a circuit breaker without a Rogowski coil as a detection unit for detecting a neutral pole current sells the circuit breaker in which the detection unit for detecting a neutral pole current is not installed and a user self-installs a current transformer and that a differential circuit section 40a is installed in addition to an integration circuit basically installed within the overcurrent relay 40.
Here, the reason why the manufacturer sells the circuit breaker without the detection unit for detecting a neutral pole current is because, if the manufacturer directly manufactures or purchases and installs the detection unit for detecting a neutral pole current to sell the circuit breaker, cost of the detection unit for detecting a neutral pole current increases due to cost for investment and maintenance of basic manufacturing facilities, resulting in an increase in overall cost of the circuit breaker. Meanwhile, a customer (who purchases the detection unit for detecting a neutral pole current as a user) may easily purchase the current transformer 1 at low cost on the market, reducing the cost of the overall circuit breaker 100, and thus, benefiting both the purchaser and user and the circuit breaker manufacturer.
The phase current detector 60 is still configured as a Rogowski coil, and thus, an integration circuit section for restoring (compensating for) a 90-degree phase delay of the Rogowski coil is essential within the overcurrent relay 40.
In particular, when an overcurrent or a ground fault current is relayed (detected and a circuit is broken), a phase should be essentially restored (or compensated), the integration circuit section is provided within the overcurrent relay 40.
In a case where the current transformer 1, rather than the Rogowski coil, is installed as a detection unit for detecting a neutral pole current, since the current transformer 1 does not have differential characteristics, namely, the characteristics of delaying a phase by 90 degrees, the integration circuit section within the overcurrent relay 40 rather acts to lead the phase of the neutral pole detection current by 90 degrees, which may cause the overcurrent relay 40 to malfunction in detecting an overcurrent or ground fault current and breaking the circuit.
Thus, in order to prevent his, in the related art illustrated in FIG. 2 comprises the differential circuit section 40a for delaying a phase of the neutral pole detection current by 90 degrees, in front of the integration circuit section within the overcurrent relay 40.
However, for the manufacturers of hundreds types of circuit breakers based on rated capacity and functions, developing and manufacturing hundreds types of new overcurrent relays due to the installation of the differential circuit section for preventing malfunction in a neutral pole current detection that may not be used according to circuit breakers may be a significant waste in cost.
Also, in a case where transformers 1 having various capacities according to various rated currents are installed, rating plugs having various internal voltage dividing resistance capacities may be additionally used to make a detection current output signal have capacity of a degree of millivolt (not shown), but even in this case, hundreds types of rating plugs should be prepared.